Telephone system



Jan. 19, 1932. c. GILLINGS TELEPHONE SYSTEM 14 Sheets-Sheet l FiledApril 14,

Charles 5min s C. GILLINGS Jan. 3%, 1932'.

TELEPHONE SYSTEM Filed April 14, 1930 14 Sheets-Sheet 2 Inventor d JChar [Es Jan. 19, 1932. c GILUNGS 1,841,725

TELEPHONE SYSTEM Filed April 14, 1930 14 Sheets-Sheet Charles 5min s%M.%

Jan. 19, 1932.

TELEPHONE SYSTEM Filed April 14, 1930 14 Sheets-Sheet 4 Inueniur CharlasBillings c. GILLHNGS Y 1,841,725

Jan. 19, 1932. c. GiLLlNGS TELEPHONE SYSTEM 14 Sheets-Sheet 5 FiledApril 14, 1930 AR rk k Q a N ua m3 ww E -'lnuaniur Charles Gillin s Jan.19, 1932. c Nss 1,841,725

TELEPHONE SYSTEM Filed April 14, 1950 14 Sheets-Sheet 6 IIW F CharlesBillings zwwg Jan. 19, 1932. Q (glLLlNGS 1,841,725

TELEPHONE SYSTEM Filed April 14, 1930 14 Sheets-Sheet '7 AB CB T Q M Ibr4 i 4 {m4 e a bad-Z InUEn UI-' Charles [fillings Jan. 19, 1932. C.GILLINGS 1,841,725

TELEPHONE SYSTEM Filed April 1 1950 7 l4 Sheets-Sheet 8 13 am GREB I YK!Charles 5min s Jan. 19, 1932. c. GILLINGS TELEPHONE SYSTEM Filed April14, 1950 14 Sheets-Sheet l0 Charla; Eiuin s Jan. 19, 1932. c GILLI'NGs1,841,725

TELEPHONE SYSTEM Filed April 14, 1930 14 Sheets-Sheet l1 IfU JETLIDFCharles Billing s Jan. 19, 1932. c. GILLINGS TELEPHONE SYSTEM FiledApril 14, 1930 14 Sheets-Sheet l3 Emil-L55 4m- Earle:

Jan. 19, 1932. c. GILLINGS TELEPHONE SYSTEM Filed April 14, 1930 14Sheets-Sheet 14 'lnusn crr Charles Billings Patented Jan. 19, 1932UNITED STATES PATENT OFFICE CHARLES GILLINGS, 0] LIVERPOOL, ENGLAND,ASSIGNOB TO ASSOCIATED TELEPHONE & TELEGRAPH COMPANY, OF CHICAGO,ILLINOIS, A. COBPOBATIONOF ILLINOIS TELEPHONE SYSTEM Application filedApril 14, 1980, Serial 1T0. 448,972, and in Great Britain Kay 28, 1929.

The present invention relates to telephone systems and is moreparticularly concerned with telephone s stems employing switches of thebackward y hunting type commonly known asfinder switches. As is wellknown, the function performed by these switches is similar to that ofswitches of the forwardly hunting type commonl known as line switches,but the contro ling arrangements must of course be difi'erent and thenumber of switches required will also be difierent. The most common useof both types of switch is for connecting a calling subscribers line toan idle one of a group of first numerical switches accessible to all thesubscribers of a group and for the sake of convenience the specificationwill refer to this particular use but it will be understood that similararrangements apply wherever a connection is to be extended from a lineto an idle one of a plurality of lines. Whilst a comparatively smallnumber of finder switches are required to deal with the callsoriginating in a group of subscribers, line switches must be providedindividual to each subscriber in the group so that a considerably largernumber are required.

Line switches, however, are comparatively cheap and simple tomanufacture and further the selector switches to which they have accesscan be arranged in comparatively large groups, for instance, groups of25, so that the number of expensive selector switches required arereduced to a minimum.' It will be realized, therefore, that if finderswitches are to prove more economical than line switches when employedto extend calls from the same group of subscribers it is necessary thatthey should be of large capacity, although it is at the same timedesirable that unduly long hunting time should be avoided. The number oflines which can be dealt with by a single level switch is limited bymechanical considerations and by the length of time required to moveover all the bankcontacts in succession and accordingly it may bedesirable to make use of switches of the two-directional t e whereby theswitch wipers may be move past groups of contacts in single short stepsuntil the required group is reached.

It is usually desirable in telephone systems that the conditionsdetermining the time taken for a calling line to be connected to a firstselector shall be dependent only on the conditions existing at that timeand shall not be dependent upon previous calls that have been set u fromthe calling line and in order to apply t is'principle it is necessar toemploy switches of the homing type, t at is to say, switches whichreturn to a home position at the end of a call, in distinction toswitches of the non-homing type, the wipers of which may stay on thecontacts last used, or may move into en agement with the next idleswitch. A possi le objection to the use of large capacit homing finderswitches is that the time ta en to reach a line which is connected tocontacts in an extreme position of the switch may be excessive, so thatthe subscribers will need to wait varying times before their line isextended to an idle first selector according to the position which theirlines happen to occupy on the banks of the switches.

It is an object of the present invention to reduce the number of finderswitches required whilst at the same time arranging that the time takenfor a calling line to be extended to an idle first selector shall besmall and substantially the same for all the subscribers. It will berealized, moreover, that the finder switch is usually linked to anexpensive selector switch so that any reduction in the number of finderswitches also provides a further economy by reducing the number ofselector switches required. In order to carry out this object ofthe'invention each group of finder switches serving each group ofsubscribers is divided into sub-groups. In order to reduce the timetaken for a calling line to be extended to an idle first selector thebanks of the subgroups are difierently wired, the bank wiring beingarranged so that when a line is calling a finder switch is alwaysstarted up in that sub-group of switches which will have the leastnumber of steps to take to connect with the calling line. This isattained according to one feature of the invention by arranging &

that in a telephone system employing a separate start relay associatedwith each subgroup of lines to which a finder switch has access astarting circuit is adapted to be extended by busy finder switchessuccessively from one switch to another, the start relays being adaptedto connect starting potential to different points in the said circuit sothat different finder switches become first choice accgrding to thestarting relay which is operate In order to reduce the total number offinder switches required, finder switches of different capacity areemployed,arranged in separate groups. According to another feature ofthe invention by which in a telephone system employing finder switchesfor establishing connections with lines over which calls are being setup, each of said lines is arranged to be accessible to finder switchesof diiferent capacity.

According to a further feature of the invention, in a telephone systememploying finder switches for establishing connections with lines overwhich calls are being set up finder switches'having different capacitiesare alloted to a group of lines the finder switches of smaller capacitybeing adapted to'be taken into use in preference to the finder switchesof larger capacity.

According to a still further feature of the invention in a telephonesystem employing finder switches for establishing connections with linesover which calls are being set up, said lines are divided into groups towhich finder switches of one capacity have access and into sub-groups towhich finder switches of a smaller capacity have access.

The invention will be better understood from a perusal of the followingspecification taken in conjunction with the accompanying drawings,comprising Figs. 1, 1A, 2, 2A, 2B, 2C, 3, 4,4(1, 5, 6, 7, 8, 9, 10 and11 which illustrate various methods of carrying the invention intoeffect. It will be understood however, that these are given b way ofexample only, and that various mo ifications may be made by thoseskilled in the art without departing from the spirit of the invention. Ageneral description of the switch layout and mechanical details willfirst begiven in connection with Fig. 1. In this example, switches ofthe two-directional step-by-step type are employed; the operation ofthese switches as is well known is effected by the operation of avertical magnet, indicated in Fig. 1, by the reference VM, whichoperates on each energization to raise the switch shaft carrying theswitch wipers one step, thereby moving the switch wipers past successivelevels of contacts until the required level is reached;-and a rotarymagnet indicated by the reference RM, which is then operated andonoperation moves-the wipers into engagement with the first-set ofcontacts in the selectedlevel, and is successively operated if necessaryto move the wi ers in a rotary direction until the wante set of contactsis found. The release rela indicated by the reference ZM is arrange onenergization to release a detent from the switch shaft which thereuponrestores to normal by re-tracing in the opposite direction its originalmotion. The switch is also provided with a vertical test wiper, shown inthe drawing by reference VW near the rotary and vertical magnets, thiswiper is arranged to engage with a separate bank of ten contacts placedvertically above one another, each contact corresponding to one level inthe regular bank. These vertical banks are shown diagrammatically at thelower left-hand corner of the drawing under references VA, VC and VB.

Referring now to the upper portion of the Figure, the apparatus shown onthe lefthand side is the calling subscribers line circuit individual tohis line, the apparatus shown in the centre is one of a group of finderswitches serving the group of lines in which the calling subscriber islocated and the apparatus on the right-hand side shows a first selectorswitch which is permanently linked to the'finder switch shown, and isincluded in a dotted rectangle. Relay A shown in the dotted rectangle inthe finder switch circuit also forms part of a selector circuit but forthe sake of convenience is shown separated therefrom.

Referring now to the ap aratus shown in the lower portion of the igure,it will be understood that this is provided in common to the group oflines under consideration. The group of finder switches required toserve this group of lines is divided into three sub-groups, these beingdesignated by the references A, B and C.

Reference will first be made to the finder allotters FAl and FAQ, eachcomprising a rotary line switch provided with five wipers adapted to beadvanced step-by-step upon each de-energization of their respectivedriving magnets DM1 and DM2. These switches are provided withtwenty-five contacts in each bank but in the present arrangement onlythe first twenty contacts are equipped. For the sake of example, the Agroup of finders is given as comprising finders 1-10, the B group ascomprising finders 2130, and the C group as comprising finders 1120. Itshould now be mentioned that the C group of finders is only taken intouse either when all the finders in the A group are busy or when all thefinders in the B group are busy. For, referring to the finder allotterFAl it will be noticed that wiper 18 extends connection from earth atoff-normal contacts similar to N1 of busy finders to the driving magnetDMl' and the driving mager; whereas the contacts correspondin to finders11-20 are permanently connecte to earth to cause the switch to ste astthese contacts and into engagement wit the first idle finder of the Agroup. A similar cir cuit is provided to magnet DM2 of the finder FA2for preselecting an idle finder in the B group and for stepping pastcontacts allotted to the finders in the C group.

' Two hundred subscribers are placed in each group and each group isdivided into ten sub-groups of twenty subscribers each. The finders inthe A and B oups have access to 100 lines each and nders in the C grouphave access to 200 lines each. The same circuit arrangements areprovided for all the finders but the connections to the bank contacts ofthe finders in each sub-group are arranged difierentl The bank contactsof the finders in the group are connected to five sub-groups of lines,the bank contacts of the finders in the B group are connected to theother five sub-groups of lines and the bank contacts of the finders inthe C oup are connected to all the sub-groups of ines.

A brief description will now be given of the C group. The finders in theC group are provided with two line banks of 200 contacts which arearranged in ten levels of ten double contacts each; two wipers beingallotted to each bank and are arranged to make contact simultaneouslyeach with a corresponding one of a double contact. The test banksimilarly consists of 200 points arranged in ten levels of ten doublecontacts each and two test wipers are provided shown in the drawing byP1 and P2, arranged so as to engage simultaneously each with acorresponding one of a double test contact. It will be seen that thesefinders have access to all the lines in the group, access being had toone hundred over one set of wipers and to the other hundred over theother set of wipers.

As will be shown later, the operation of a finder switch is initiated byearth connected to a common start lead similar to lead 16 shownconnected to level 3 of the vertical bank of the finders of the C group.

When a call is initiated earth is connected to that contact of thevertical bank which corresponds to the level to which the callingsubscribers line is connected. At the same time, the operation of thestart relay'AX is eiiected for setting in operation the finder which isto connect with the calling line.

Interaction then takes place between magnet RM and relay G of thehunting finder until wiper VK encounters earth on the marked contact ofthe vertical bank. When this level is reached relay T operates todisconnect the vertical magnet and connect up the rotary magnetwhereupon inter-action takes place between relay G and magnet RM torotatethe wipers over the selected level. Simultaneous hunting thentakes place over wipers P1 and P2 until the calling line is found.

The operation of a finder in the C group in this manner, however, on]takes place under conditions of heavy tra c and in general, use will beof a finder in the A group or in the B group according as to whether thecalling line is located in sub-groups 15 or in sub-groups 6-10. In thecase of the finders in the A and B groups two sets of wipers areemployed in the same manner as in the C oup. but since only five levelsare required it is possible to dispensevwith one line bank and toarrange for both sets of line wipers to en age wit the other line bank,the wipers then in arranged five steps apart. In the case of t e A groupof finders, one set of wipers is arranged to engage with ten lines ineach of the sub-groups 1-.5 and the other set of wipers is arranged toconnect with the other ten lines in those sub-grou s. In the same way,the line wipers in the group of finders are arranged to have access in asingle bank to calling lines in sub-groups 610. This arran ement is forthe sake of economy, although i? switch mechanism and bank arrangementsfor all the finder switches this may readily be done by leaving theupper five levels in each bank une nipped. It will be noticed that earthis s own directly connected to the fifth vertical bank contact of groupsA and B, and to the tenth vertical bank contact of group C. This isprovided so that in the case of a false call being initiated due to afault then the finder set in operation will not be operated past theselevels.

Referrin now to the bottom right-hand corner of Fig. 1, relay AB isprovided in common to finders of the A group, and is normally energizedas long as an idle finder is available in this group. Similarly relay BBcommon to finders of the B group and relay CB common to finders of the Cgroup are also normally energized. In order that the circuits includingarmatures of these relays may be more readily understood these armatureshave been placed either in a vertical position or in a horizontalposition in the drawing in order to distinguish them from the armaturesof other relays," these latter being shown in a slanting position.

When all the finders in the A group become busy, relay AB releases andcauses relay AY to operate, therebv at armature (lg 3 causing the finderallottcr FAI to select an idle finder in the C group. In a similarmanner when all the finders in the B group become busy, relay BY4 isoperated to cause the finder allotter FAQ to select an idle finder inthe it is desirable to use the same C group. By referring to the crossconnecseen 'that under normal conditions a maximum of five verticalsteps is necessary to reach the required level, and that 610 steps areonly required if a finder in a common group is taken into use.

In the arrangements shown in Fig. 1a the finders in the common group arefurther subdivided into two groups and the connections to the allottersFAl and FA2 are arranged so that if the callin line is in sub-groups 1-5and all the fin ers in the A group are busy the finders in the commongrou that will generally first be taken into use W1 1 also only have tobe operated to a maximum of five vertical steps, and only, therefore,when the finder allotter selects finders 16-20 will it be necessary formore than five vertical steps to be taken. Similar arrangements are alsomade for the interworking of the B and C groups.

Referring now to Fig. 2, the circuit arrangements of the finders may besimilar to those shown in Fig. -1, but the vertical bank contacts arenot provided with the reversal connections shown in Figs. 1 and 1A, thatis to say, no arrangements are provided for reducing the time requiredfor a finder to establish connection with the lines of subscribers insub-groups 610. The finders in the A and B groups each have access to100 lines but in this case only one set of wipers is provided and oneline bank of 10 levels.

The finders in the A group may be considered as having access tosubscribers having an odd hundred digit and finders in the B group ashaving access to subscribers having an even hundreds digit. The findersin the C group have access to 200 lines and are provided with two setsof wipers over which simultaneous hunting takes place. If all thefinders in the A group become busy,-relay 0A is operated to extend thestarting connection through to the vertical bank of the common group,and similarly if all the finders in the B group become busy, relay EB isoperated to extend the starting connection through in a similar manner.

A further modification employing similar arrangements is to provide thefinders of the common group with two vertical test wipers and to arrangethe operation of the allfinders-busy relay of the A group or the B groupto cause the selection of one set of wipers of the finder of the commongroup taken into use so that simultaneous hunting is not then necessary.The arrangement of the vertical banks is then as shown in Fig. 2C, thecircuits of the finders in the A and B groups are as shown in Fig. 2aand the cir-' cuits of the finders in the C group are as shown in- Fig.2b. The finders in the A and B groups have access to 100 lines, and areprovided with only one set of wipers. The circuit arrangement of thesefinders is similar to that shown in Fig. 1, but has been slightlymodified as shown in Fig. 26, by dispensing with the relay S which isarranged to change over connections from one set of wipers to the other.The arrangement of the finder allotters is similar to that alreadydescribed and these are shown in Fig. 2?), but in order to avoidcomplicating the drawin the connections to the banks of the fin erallotter FA2 have been omitted.

The finders employed in the common grou have access to 200 lines and arefitted wit two sets of wipers. The circuit arrangement again is similarto that shown in Fig. 1, but has been modified as shown in Fig. 2?; byarrangin that when a finder is taken into use by a calling subscribernormally served by a finder in the A group, that is to say, when thefinder is taken into use over the wipers of the finder allotter FA1, thediscriminating relay D is operated before hunting takes lace and remainslocked operated while the der is in use. The operation of this relayconnects up the set of wipers including wiper P2 and in addition changesover the circuit of the right-hand winding of relay T from verticalwiper YWl to vertical wiper VW2. If, however, the finder in the commoncup is taken into use by a calling subscriber normally served by afinder in the B group, that is to say, when the finder is taken into useover the wipers of the finder allotter FAQ, earth is not connectedforward to operate relay D and hence wiper W1 and wiper P1 with itsassociated line wipers will be employed.

Referring'now to Figs. 3 and 4, these show a finder system employingfinders similar to those employed in the system shown in Fig. 1, butarranged with relay distribution, that is to say, relays are provided totake the place of the rotary line switch finders allotters FAl and FA2.The circuit shown in Fig. 3 is similar to that shown in Fig. 1, with theexception of the provision of relay HR and the circuit shown in thelower portion of the fi'ginto five sub-groups and the finders in the Cgroup are divided into ten sub-groups; the finders in each sub-group arenormally operated to connect with calling lines terminating in theirfirst level.

Referring to Fig. 4, the A group of finders AG is shown-diagrammatically divided into five sub-groups L1, L2, L3, L4 and L5,and the finders in each: sub-group are numbered from F1 to'FN. It willbe seen from Fig. 3 that if the calling subscriber is located in level 1of the A group, relay AX will be operated and at armature awl. will markthe first vertical bank contact and at armature aw2 will extend earth toarmature hr3 of the first finder in the first sub-group. This finderwill then be operated one step vertically to select the wanted line.

If all the finders in the first sub-group are busy, the earth connectionwill be extended to the first idle finder in the second subgroup. Thebank multiplying to the finders in this group is arranged so that onlyone vertical etc is necessary to connect with subscribers in the secondsub-group but two vertical steps are necessary to connect withsubscribers in the first sub-group. If, however, all the finders in thissub-group are busy, the earth connection will be extended to the thirdsubrou and if an idle finder is available it wi l ta e three verticalsteps to connect with the calling level. Similarly, if successivesub-groups are busy, the successive finders available in the sub-groupswill take further vertical steps to select the calling level.

In the majority of cases, however, a finder will be available in thefirst sub-group. and only one step will be necessary, the generalprinciple of operation being that the large number of steps areonlynecessary under exceptional circumstances, the number of steps becomingprogressively larger as the probability that the corresponding switchwill be taken into use becomes less.

As already mentioned, it the finder set in operation is in the sub-groupcorresponding to the sub-grou in which the calllng subscriber is locateonly one vertical step will be necessary. This may be furtherillustrated by relating to the operation of relay AZ, which takes placewhen a subscriber in the fifth sub-group is calling; under conditions oflight trafiic a finder in sub-group L5 will then be operated and as theline bank contacts corresponding to the fifth vertical bank contact aremultiplied to the first level in the banks of finders in group L5 onlyone vertical step is necessary. Similar arrangements are also made inthe R and C groups. If all the finders in grou A become busy relay ABwill be operate and will extend the starting circuits, to the commongroup of finders. Similarly, if all the finders in the B oup becomebusy, relay BB will be operated and will extend the starting circuits tothe'common roup of finders.

Referrin now to igs. 5 and 6, these show a modification of the circuitsof the finders arranged so that the finders instead of hunting in arotary direction for the absence of earth potential, will hunt untilbattery potential is encountered on the private contact of the callingline. The arrangement of the finder allotters and vertical bank contactsis also somewhat difierent from that shown in Fig. 1, but it will beunderstood that the arrangement of the finder allotters and verticalbank contacts shown in Fig. 1 may be used in this case also.

Referring now to Figs. 7 and 8, these show a modification of Figs. 5 and6, the number of relays necessary havin been reduced by combining thestepping re ays of the selector and the finder an by arrangin that therelay which operates when the callin level is reached also acts as aguard relay or the selector. For the sake of convenience, the finderallotters and vertical bank connections are identical with that shown inFigs. 5 and 6, but it will be understood that the circuits of the finderselector link may be used with any other kind of allotting arrangements.

Referring now to Fig. 9, this shows a finder having access to 200 linesand arranged to hunt simultaneously in two levels in which a shunt fieldrelay is employed as a stepping il-elay in conjunction with twoswitching reays.

Referring now in Figs. 10 and 11, this shows an arrangement for useunder exceptionally heavy traiiic conditions where it may be necessaryto provide up to 50 finders to deal with the traffic from a group of 200subscribers. It will be realized that this example represents the limitof that shown in Fig. 1, for by increasing the number of finders itbecomes necessary to reduce the number of finders in the common group ifuse is still to be made of 25-point allotting switches and in the limitshown in Figs. 10 and 11 the number of finders in the common grou isreduced to zero. If all the finders accessible to either finder allottershould become busy, finders in the other group are taken into use and ifeither of the finder allotters develops a fault the attendant by theoperation of a key. is able to switch in an auxiliary finder allotter.

Having now given a general description of a layout of the switches, andtheir operation, a detailed description of the operation of the circuitswill now be given.

Referring now to Fig. 1, it will be assumed that conductors 10 and 11terminate at the substation of a subscriber in sub-group 3 and alsoterminate at the third set of bank contacts in the third levelaccessible to the upper set of wipers. When the subscriber initiates acall by removing his receiver from the switch hook, a loop is completedacross conductors 10 and 11, thereby completing circuit for theleft-hand windin of relay LR as follows: battery, left-han windin ofrelay LR, armature 001, conductor 10, t rough a loop circuit at thecalling subscribers substation, conductor 11, armature 002, to earth.

Relay LR thereupon operates, at armature Zrl disconnects earth "from theprivate bank contact terminating a line of the calling subscriber inorder to mark this contact as that of the calling line and at armatureZ12 connects earth to the start lead 16, which is common to all thetwenty subscribers in sub-group 3. A circuit is thereby completed forrelay in value the earth potential connected through AX throughresistance 12 and relay AX thereupon operates. At the same time, directearth is connected to the'third vertical bank contact of the finders inthe A group, and the finders is the C group.

'As the resistance 12 is comparatively high two of these resistances inseries to the other vertical bank contact is insufiicient "to mark thesebank contacts to indicate that the callin subscriber is located in thecorrespondin su -group. Upon the 0 eration of relay A a circuit iscompleted or relay AD as follows: earth, armature azvl, armature ae2,winding of relay AD, armature a 1, interrupter springs of driving ma etD 1, winding of driving magnet D 1 to battery. Magnet DMl does notenergize in this circuit, owing to the relatively high resistance ofrelay AD, but relay AD operates and at armature ad2 connects up earth tostart the operation of a finder. At armature (m2, relay AX closes a1point in the circuit of the overflow meter 0 which is employed toindicate the number of calls initiated during periods when all thefinders are busy.

It will be assumed first of all that the wipers of the finder allotterFAl are resting in engagement with bank contacts corresponding to a freefinder in the A group, and that this is the finder shown in the upperportion of Fig. 1. At armature ad3 a circuit is completed for the upperwinding of relay A as follows: earth, resistance 14, armature ael,armature ad3, wiper 15 and the contact on which it is resting, upperwinding of relay A to battery. Relay A operates, at armature a1 opens aoint in the circuit of the release magnet Z and at armature a2 completesa circuit for relay B which in turn operates, at armature b1 connectsearth to the release trunk conductor and at armature 52 opens a furtherpoint in the circuit of the release magnet ZM. Earth at armature 61 alsoextends over the normally closed ofi-normal springs N1 and the bankcontact engaged by wipers thereby closing a point in the circuit ofmagnet DMl, this circuit, however, is maintained open at armature adl.At armature ad2 earth is extended over wiper 13 and the bank contact onwhich it is resting, back contact of armature t2 and said armature,interrupter spfings V controlled by the vertical magnet M, interrupter srings R controlled by the rotary magnet R winding of stepping relay G tobattery. Relay op- 4 erates in this circuit and at armature g2 completesa circuit for vertical ma net VM as follows: earth, armature ad2, wiper13 and the bank contact on which it is resting, armature g2, armature t4and its back contact, winding of vertical magnet VM to battery. Verticalmagnet VM thereupon energizes and upon energizing raises the switchshaft and wipers one vertical step, thereby bringcontact has been markedby the connection,

of earth thereto, the potential encountered on the first vertical bankcontact by wiper,

VW will be insuflicient to cause any operation to take place in thefinder. Shortly after wiper VW has been moved into engagement with thefirst vertical bank contact the interrupter springs V are opened therebyopening the circuit of relay G, which thereupon de-energizes and atarmature g2 opens the circuit of the vertical magnet VM. Magnet VMthereupon de-energizes and allows the interrupter springs V to open.

The circuit previously traced for relay G is then again completed, relayG again operates to close the circuit of the vertical mag net VM, whichthereupon operates to step the vertical wiper VW into engagement withthe second vertical bank contact. Since, however, direct earth is notencountered, the operation of the finder continues as before, and thevertical magnet is again energized to step the vertical wiper intoengagement with the third vertical bank contact, where it encountersdirect earth which short-cir-.

cuits the right-hand winding of relay T. Shortly after the verticalwiper VW has stepped into engagement with the third ver tical bankcontact, the interrupter springs V are opened and the following circuitbecomes effective: earth, armature 1T2, armature 003, conductor 16,third vertical bank contact, vertical wiper VW, rotary off-normalsprings RON, right-hand winding of relay T, rotary interrupter springsR, winding of relay G to battery. Relay G is maintained operated in thiscircuit whilst relay T operates and at armature t1 completes a lockingcircuit for itself over its left-hand winding to earth at armature ad2.At armature t4, relay T opens the circuit of the vertical magnet VM andcompletes an operating circuit for the rotary magnet RM as follows:earth, armature ad2, wiper 13 and bank contact on which it is resting,armature g2, armature t4 and its front contact, winding of magnet RM tobattery. Magnet RM thereupon operates and rotates theswitch wipers intoengagement with the first set of bank contacts in theselected level.

The rotary off-normal springs RON are opened as soon as the switch shaftbegins to rotate, thereby disconnecting the vertical

